The floor of an automotive body (hereinafter simply referred to as “floor”) has rigidity to primarily resist the torsion and bending of the vehicle body when driving the vehicle, and also transfers an impact load in a case of collision of the vehicle. The floor also affects a weight of the automotive body significantly. Accordingly, the floor is required to have mutually contradicting properties, that is, a high rigidity and a lightweight. The floor includes flat panels that are joined to each other by welding, vehicle widthwise members that have substantially gutter-shaped cross sections and are fixed to the flat panels along the vehicle widthwise direction, and vehicle longitudinal members that have substantially gutter-shaped cross sections and are fixed to the flat panels along the front-back direction of the vehicle body.
The flat panels include, for example, a dash panel, a front floor panel, a rear floor panel, and the like. The vehicle widthwise members are members fixed by welding and disposed along the vehicle widthwise direction of these flat panels to increase the rigidity and strength of the floor. The vehicle widthwise members include, for example, floor cross members, seat cross members, and the like. The vehicle longitudinal members are members fixed by welding and disposed along the front-back direction of an automotive body to increase the rigidity and strength of the floor. The outward flangevehicle longitudinal members include, for example, side sills, side members, and the like. Among them, reinforcing members such as the vehicle widthwise members and the vehicle longitudinal members are typically joined to other members via outward flanges formed at ends of the reinforcing members. For example, a floor cross member, which is an example of the vehicle widthwise members, is joined to the tunnel portion of a front floor panel and to a side sill via outward flanges that are formed at both ends of the floor cross member.
FIGS. 19 (a) and 19 (b) illustrate a floor cross member 1, which is a representative example of a member joined to other members with outward flanges 4 formed at both ends in the longitudinal direction of the member. FIG. 19 (a) is a perspective view of the floor cross member 1 and FIG. 19 (b) is a view on the arrow A in FIG. 19 (a).
A front floor panel 2 is reinforced, for example, by a tunnel portion (not shown) that is joined to the upper surface (indoor-side surface) of the front floor panel 2, and also by a side sill 3 and the floor cross member 1. The tunnel portion is a structural member projecting toward the inside of a vehicle along the substantially widthwise center of the front floor panel 2. The side sill 3 is spot welded to the upper surface of the front floor panel 2 at each widthwise edge of the front floor panel 2. Both ends of the floor cross member 1 are spot welded to the tunnel portion and the side sill 3 with the outward flanges 4 formed at both ends in the longitudinal direction. This improves the rigidity of the floor and the load transfer property when an impact load is applied.
As described above, the floor cross member 1 is an important structural member to perform a function to improve the rigidity of an automotive body and to absorb an impact load in a case of a lateral collision event. Accordingly, in an aim to reduce body weight and improve collision safety, a high-tensile steel sheet of smaller thickness and larger strength, such as, for example, a high-tensile steel sheet having a tensile strength of 390 MPa or more (high-strength steel sheet or high-tensile strength steel sheet), has been used as a material for the floor cross member 1 in recent years. However, there is still a strong demand for a floor cross member 1 that has more improved load transfer property when an impact load is applied. To address the demand, it is necessary to improve the load transfer property when an impact load is applied, not only by increasing the material strength alone but also by modifying the shape of the floor cross member 1.
Although Patent Literatures 1 to 3 do not intend to form a floor cross member, Patent Literatures 1 to 3 disclose inventions to solve defects in shape fixation of press-formed products made of high strength materials by modifying pad mechanisms used with dies. These inventions have attempted to make an improvement in the shape fixability after press forming by intentionally generating deflection of a material during forming depending on the positional relationship between the top of a punch and a flat pad of only a part that faces a flat part of the top of the punch.